Troponin complex (troponin T (TnT), troponin I (TnI), troponin C (TnC)) plays an important role in the Ca2+ -dependent regulation of muscle contraction. To study the proximity relationships of the troponin T-I-C subunits, a series of additional mutation of TnT and 25 K fragment of TnT have been made, expressed in E.coli and purified. The single-Tap mutant K fragment of TnT (TnT/25 K W265F) was needed as the donor for distance measurements by fluorescence energy transfer. Series of donor-acceptor pairs were chosen and the spectroscopic properties measured under different buffer conditions. The distances between Tap-212 of TnT/K to Tin-Cys-96 (N96C), Tin-Cys-117 (S117C), TnC-Met-25 and TnC-Cys-98 were measured. The distance measurements are being used to develop models for muscle contraction.[unreadable]